He advised the Queen to prorogue Parliament knowing full well that it was unlawful to do so. He wanted Parliament suspended because he couldn't get the House to agree to his Brexit plans. Remember he said he would rather be dead in a ditch than miss his October Brexit deadline, he knew he would miss it so he tried to shut down Parliament to stop the debate. The High Court ruled that the effect of a lengthy, parliamentary suspension at this time was unlawful.
Giving judgment,
Lady Hale said: “The courts have exercised a supervisory jurisdiction over the lawfulness of acts of the government for centuries.”
They then ruled the decision to prorogue parliament was unlawful because it had “the effect of frustrating or preventing the ability of parliament to carry out its constitutional functions without reasonable justification”. She added: “No justification for taking action with such an extreme effect has been put before the court.”
Following the judgement the court said the speakers of both the Houses of Lords and Commons could reconvene immediately.